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Thursday, May 26, 2022

Here’s How Martian Explorers Could Best Produce Electricity on the Red Planet

Artistic conception of a crewed mission to Mars.

Artistic conception of a crewed mission to Mars.
Image: NASA

Scientists are on the fence as to whether solar or nuclear should be the preferred source of power for small teams visiting the Martian surface. New research suggests both options are good, with geographical location being the determining factor.

The researchers compared two different power-generating options for a crewed trip to Mars: solar cells and nuclear power from small fission reactors. A key consideration was the amount of weight, or “carry-along mass,” required to build each solution, as missions to Mars will seek to pack in the most efficient way possible. The results, published today in Frontiers in Astronomy and Space Sciences, suggest both options are viable, but with a rather important caveat having to do with geography.

“The main result was that which one ‘wins’ depends on the location on Mars,” Anthony Abel, a researcher from the Department of Chemical Engineering at UC Berkeley and a co-author of the study, explained in an email. “The overall result was that nearer the equator, solar was better than nuclear, while nearer the poles, nuclear was better than solar.”

This is good information to have, as it could have significant bearing on the type of power-generating devices that each future mission will want to bring to Mars. NASA should take note, as the space agency is planning to send the first crewed mission to planet in the late 2030s or early 2040s. That said, these findings are specific to a six-person crew on a 480-day mission to the Martian surface (the first missions won’t likely last longer than 30 days), but the researchers say their results could speak to even larger and longer missions, including permanent colonies. Aaron Berliner from the UC Berkeley Department of Nuclear Engineering is a co-author on the study.

Future explorers will need electricity to support their ground missions. This power will be needed to generate warmth, oxygen, and clean drinking water, as well as to also power more advanced activities, such as LEDs to shine on crops and 3D printers to churn out useful parts. Abel and Berliner, as members of the Center for the Utilization of Biological Engineering in Space (CUBES), have a vested interest in this subject, as their imagined concepts will depend on sustained power to work, such as the use of microbes to produce plastics and pharmaceuticals. Abel and Berliner wanted to know how to best provide power to their future space-enabling systems, leading to the new study.

“We knew that rovers in the past had been powered by either solar cells or nuclear power generators, and that both solar and nuclear had been proposed for crewed missions to Mars,” Abel told me. “Nuclear generators will work more or less the same regardless of where you are, but solar cells will operate pretty differently because sunlight is the source of power.”

The consistency of nuclear and the tenuousness of solar has led some experts to suggest that nuclear might be the smarter, more reliable choice. Indeed, there are many factors to consider when it comes to generating solar power on the Red Planet. Mars, in addition to being farther away from the Sun than Earth, is colder, dustier, and dryer. Abel and Berliner had to take these factors into account, calculating variations in solar intensity, mapping out surface temperatures, and analyzing the way gasses and particles absorb and scatter light on Mars, as all of this influences solar cells’ ability to produce power.

“So, we needed to model the Martian atmosphere to figure out how much light would land on our solar cells, and then model the solar cells to figure out how much power they would generate,” Abel said. “The Sun also sets every day, so when using solar, you have to figure out how to store energy to ‘keep the lights on’ at night.”

Equipped with this data, the team then calculated the carry-along mass of the different energy solutions—the “amount of stuff we would need to bring with us from Earth to Mars,” Abel said, adding that “less is better.” This led the team to conclude that solar works better nearer the equator, while nuclear makes more sense near the poles.

Indeed, while a miniature nuclear fission device operates the same regardless of the chosen location on Mars, the same cannot be said for solar. A photovoltaic array that uses compressed hydrogen for energy storage was calculated to have a carry-on mass of 8.3 tons at the Martian equator, compared to 9.5 tons for the equivalent nuclear option. But as the efficiency of solar decreases with distance to the equator, our intrepid explorers would need to pack 22 tons of material to build an equally efficient solar power array at the Martian poles. And future explorers will certainly want to visit the poles, as these regions are likely to have valuable water ice.

The primary takeaway of the research is that “both solar and nuclear can work, but it depends on where you land, how many people go, and how you store energy,” said Abel. Interestingly, the Martian surface is roughly split down the middle in terms of whether solar or nuclear would be the ideal power option. In terms of energy storage, the team found that it would be best to take excess electricity and use it to split water molecules into hydrogen and oxygen.

“Those gasses can be stored easily in tanks until the nighttime, when the solar panels aren’t producing energy. Then, we use a fuel cell to release the energy stored in those gasses back into electricity, regenerating water,” Abel told me. “You’ve probably heard of fuel cell buses, which rely on the same technology to power their engines.”

I asked Abel if these findings might be transferable to Mars missions lasting longer than 480 days and involving more than six people.

“Things will be a little bit different for bigger missions or for a colony,” he responded. “Because the habitats will be bigger, they’ll need more power, so your power generation system will also need to get bigger. For solar, your energy storage system will also need to be bigger, which might put solar at a bit of a disadvantage.”

That said, Abel believes these findings could translate well to other mission types. Once a landing site is chosen and the number of crew members selected, mission planners “could use our calculations to determine if nuclear or solar will be better at that site for that size of mission.”

According to Abel, solar would be better for a mission to Jezero Crater, the landing site of NASA’s Perseverance rover, while nuclear would be the superior option at Utopia Planitia, where the Viking 2 rover landed. These results “might change for bigger missions, but redoing the calculation for different mission sizes is pretty easy now that we can predict how much power solar cells can generate in a given place on Mars,” he added.

Looking ahead, the team will work to determine how much food, medicine, and other resources will be required by Martian ground crews, and how many and what type of solar panels would have to support those needs. They’re also hoping to design mission plans that take brighter days or the summer months into account, during which time Martian explorers could store materials for use during the winter, when sunlight is less intense.

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